The present invention relates to improved additives for drilling fluids used in the drilling of oil, gas and geothermal wells in high-temperature subsurface strata or formations of the earth.
For drilling wells to extract or exploit petroleum, natural gas, geothermal fluids, underground water or other fluid minerals naturally existing in the subsurface of the earth, a so-called rotary drilling process using a drilling fluid or mud is generally applied.
In the rotary drilling, the drilling fluid is pumped into a drill pipe to be spouted out from bit nozzles for removing cuttings from the face under the bit. More specifically, the drilling fluid cools and lubricates the bit and drill stem, protects against blowouts by holding back subsurface pressures, and deposits a mud cake on the wall of the borehole to prevent loss of fluids to the formation or influx of subsurface fluids into the borehole, in addition to its function of bringing the cuttings to the surface as they are removed from the face under the bit.
As such a drilling fluid or mud playing these important roles in the rotary drilling, a water-clay based mud is typically used which is an aqueous suspension of a clay conditioned with several additives. As such a clay, commonly used as bentonite which is a colloidal clay, mainly composed of montmorillonite, which swells when wet.
Among various additives for conditioning such bentonite-type drilling muds, polyphosphoric acids, sodium lignosulfonate, chromium iron lignosulfonates, sodium humate, sodium chromohumate, combinations of lignin and humic acid or the like substances are used as dispersants or deflocculating agents.
However, since drilling fluids or muds conditioned with such prior art dispersants are thermally weak, they undergo such a remarkable gelation that the bit gets clogged and the drilling rate is substantially reduced, when used for drilling in high-temperature formations. The gelation referred to herein denotes a state of a drilling fluid in which the colloidal particles of the clay, namely, its main constituent, lose their dispersability or deflocculatability to turn into a jellylike state or to be semisolidified with a substantial loss of fluidity or the mud.
Further, if the circulation of the drilling fluid is suspended because of troubles or by some other causes, the drilling fluid may gel to such an extent that the rotary motion of the drill pipe is stuck. Even if the drill pipe is not restricted, a thick mud cake or the wall of the wellbore may be peeled off to restrict the drill pipe because a strong force will be applied suddenly on the wall of the wellbore when the drilling is started again with the drill pipe surrounded by such gelled mud. While, if Dyna-Drill, a downhole motor driven by a drilling fluid that imparts rotary motion to a drilling bit connected to the tool, is used for drilling, the drilling fluid must be continuously controlled to specified high fluidity within particularly narrow limits.
As a drilling fluid additive that does not lose the stability even in the drilling of wells in high-temperature formations, Japanese Patent Publication No. 53(1978)-35,875 discloses combinations of alkali metal salts of humic acid and mineral bitumen. However, experiments by the inventors have revealed that these patented additives have a heat resistance up to about 180.degree. C., but they are gradually deteriorated at higher temperatures to lose their functions almost completely at about 200.degree. C.
Besides these, Japanese Patent Publication No. 53(1978)-33,953 and British Pat. No. 1,276,411 disclose alternative drilling fluid additives with the former being directed to additives comprising sulfonated lignite or sulfonated humic acid and complex compounds of phenol condensates and transition metals and the latter to additives comprising a mixture of at least one alkali salt of sulfonated lignite or sulfonated humic acid and at least one metal chelate thereof. In these prior art literature, the lignite generically means various carboniferous materials graded intermediate the peat and the bituminous coal and, more specifically, such coalful materials containing at least about 40%, ordinarily about 50% to 60% by weight of humic acid as dry.
The teaching of the aforecited Japanese Patent Publication No. 53(1978)-33,953 and British Pat. No. 1,276,411 are characterized by the use of sulfonated lignite or sulfonated humic acid with a resultant reduction in the water loss or undesirable migration of the liquid part of the mud into a formation and improvement in gel strength as well as in thermal stability.
However, elevated temperature aging tests have shown that the foregoing prior art drilling fluid additives begin to show deterioration in their functions at about 180.degree. C. to become unusable practically at about 220.degree. C.
Although these facts will not give rise to any problem so long as normal drilling operations concerned.
In ordinary drilling operations, the drilling fluid pumped into the drill pipe is spouted out of the bit nozzle and rises up to the surface through the annulus between the wall of the wellbore and the casing. After separating the cuttings on the shale shaker on the surface, it is pumped again into the drill pipe for a recycled use if its temperature is not so high. However, if its temperatue is elevated (ordinarily at temperatures higher than about 60.degree. C.), the drilling fluid is cooled before recirculation.
Therefore, no difficulties will be encountered in using drilling fluids conditioned with the aforementioned prior art additives so long as the drilling is continued under normal conditiones.
However, if the drilling is suspended for replacement of the drill bit or because of troubles of the drilling equipment, the circulation of the drilling fluid is stopped. Then, the drilling fluid is heated by the geothermal heat to gel itself because the drilling fluid additive contained therein will lose its functions.
Also, when a casing pipe is to be cemented, the drilling operation is suspended until the cement is hardened, and the drilling fluid will undergo deterioration in this cementing period.
Further, if the drilling fluid is once exposed to elevated temperatures due to a prolonged stay in a wellbore, it cannot be used for recirculation any more even if it is not severely affected to gel. Thus, after a prolonged suspension of the drilling operation, the semisolidified mud column must be removed from the wellbore, which is a laborious and time-consuming work.
While, in the drilling of a geothermal well, it is customary that the drilling operation is suspended for 36 to 72 hours for the measurement of the temperature distribution in the formations. Thus, if the drilling fluid gels during such a prolonged suspension of the drilling operation, it becomes impossible to hang down a temperature sensor in the wellbore.
As petroleum, natural gas or geothermal fluids are exploited from deeper points of the earth in recent years than ever, the formation pressure and geothermal temperature increase significantly, and it has become practically impossible for the aforementioned prior art drilling fluid additives to provide satisfactory drilling fluid conditioning functions and properties such as deflocculating effect, clay swell suppression effect and thermal stability. Particularly, when used in the drilling at these elevated temperatures as encountered in the deeper points of the earth, they show such a remarkable reduction in their water loss preventing function that a large amount of newly prepared mud must be added. Especially, in those wells deeper than 1,000 m, the volume of the drilling fluid undergoing deterioration by geothermal heat will amount to several hundred kiloliters in total. Thus, if such a large quantity of the drilling fluid is to be discarded, the drilling operation will be delayed by the days required for conditioning a new drilling fluid, in addition to a great economical disadvantage.
Under these circumstances, the present invention has been achieved with a view to overcoming the aforementioned drawbacks of the prior art drilling fluid additives.